The present invention relates to a driving device and method for driving electrostatic actuators. More particularly the invention relates to a driving device and method using a compounded AC signal.
Electrostatic actuators have become selected and are the solution of choice for actuators that employ low power, operate at high speed, require low cost to produce, and are of small size. These devices present significant advantages: over thermal devices by requiring much less power; over electromagnetic devices using less power and having smaller size; or piezoelectric actuators that have a higher cost and have a much smaller amplitude of motion.
To date, however, there are no commercially available electrostatic actuators. Of particular concern are electrostatic actuation in the presence of dielectrically isolated electrodes, where specific problems are incurred.
In electrostatic actuators, the desired displacement is the result of the attractive electrostatic force generated by the interaction between a distribution of opposite sign charges placed on two bodies, one of which is moveable. For the purposes of this invention, these two bodies are known as actuator plates. The actuator plates are placed apart by a predetermined distance. The charge distribution is then generated by applying a potential difference between two conductive electrodes that are part of the actuator plates. The actuator will be in the ON state or mode when a potential difference is applied between the electrodes and the two plates move toward each other. The actuator will be in the OFF state when the electrodes are at the same potential (shorted).
A DC voltage is theoretically capable of inducing the xe2x80x9cONxe2x80x9d state of the actuation cycle. Practical problems, however, limit the application of a DC voltage for the actuation of some electrostatic actuators. In real devices, DC driving shows memory effects, such that the behavior of the actuators depends strongly on the history of the actuation process. DC driving also induces stiction (1) through charges injected in the dielectric at the dielectric/metal electrode interface, and (2) through charges trapped at the dielectric/air interface.
Using an AC signal for inducing the ON state of an electrostatic actuator is also possible and it can remove the drawbacks of the DC driving.
Sine wave AC drive has been considered and found to have some detrimental properties. Sine wave AC drive does not generate the steady level of electrostatic pressure required in many applications. The displacement of the actuator plate/plates tends to follow the sine wave applied voltage, resulting in an undesired vibratory motion when the AC sine wave voltage is applied. To overcome this drawback a multiphase/multielectrode driving scheme can be used. This method results however in complicated structures and limited force. Square wave AC driving can provide high and steady electrostatic forces. However, it can also produce premature stiction that adversely affects performance sooner than would permit many practical devices to operate with reasonable life expectancy.
One family of patents describes fluid control employing micro miniature valves, sensors and other components using a main passage between one inlet and exit port and additionally a servo passage between inlet and outlet ports. The servo passage is controlled by a control flow tube such that tabs are moved electrostatically. U.S. Pat. No. 5,176,358 to Bonne et al teaches such a fluid regulating device, while divisional U.S. Pat. Nos. 5,323,999 and 5,441,597 relate to alternative embodiments.
The actual electrostatic device is only briefly described in the above patents, wherein at least one tab formed as part of a dielectric layer moves toward and away from an aperture upon activation of a means for varying the potential of at least one electrode associated therewith to generate an electrostatic force.
The above referenced patents identify another family of patents for further information on microvalves using electrostatic forces. The pending U.S. patent application referred to in those first discussed patents has matured into U.S. Pat. No. 5,082,242 to Bonne et al. This patent describes a microvalve that is an integral structure made on one piece of silicon such that the device is a flow through valve with inlet and outlet on opposite sides of the silicon wafer. The valves are closed by contact with a valve seat where surfaces must be matched in order to avoid degradation of valve performance. Two patents, U.S. Pat. Nos. 5,180,623 and 5,244,527 are divisional patents relating to the first patent. These patents generally describe operation of the electrostatic valve as being driven by various kinds of voltage sources. Specifically, the valve is said to operate as a two position valve with fully open and fully closed positions by applying a DC voltage between electrodes. Also, operation as a proportional control valve is disclosed as being effected by applying a voltage proportional to the voltage necessary to close the valve. Finally, These patents describe operation of the valve with a pulse width modulated voltage signal to modulate gas flow through the valve.
In some electrostatic actuators, the actuator plates have to come in intimate contact during the normal operation cycle. These actuators are sometimes referred to as touch-mode electrostatic actuators. In order to prevent electrical shorting during the touch phase of the operation cycle, the conductive electrodes are isolated from each other by dielectric layers. In order to get the maximum work from a specific device, large electric fields are usually developed between the two conductive electrodes. The non-linear character of the electrostatic attraction results in a snapping action, where the actuator plates move toward each other with accelerations as high as 108 g and speeds that exceed 103 m/sec. After the impact, the free surfaces of the actuator plates are pushed against each other by the large electrostatically generated pressure. This operation mode creates the possibility of very large mechanical impact and strong interaction forces being developed between the actuator plates. Some of these forces can continue to act after removal of the potential difference between the actuator plates. In some cases, these forces are stronger than the restoring forces available for bringing the electrodes in their original position. In such a case, the two electrodes remain temporarily or permanently attached and the actuator stops functioning as intended and desired. This condition is sometimes referred to as xe2x80x98stiction.xe2x80x99
The main forces producing stiction in electrostatic actuators are surface interaction forces (solid bridging, Van der Waals forces, hydrogen bonds) and electrostatic forces produced by charges permanently or temporarily trapped into the dielectrics. To reduce the surface interaction forces, two approaches may be used. The first, reducing the contact area, requires more sophisticated structures and gives up some of the available electrostatic force. The second, reducing the surface energy of the layers in contact, has not yet been successfully demonstrated for devices based on that concept.
During the lifetime of an actuator, the large mechanical impact and the large electrostatic pressure can gradually increase the real contact area between the actuator plates and enhance the surface interaction forces.
It would be of great advantage to the art if this buildup could be reduced or avoided altogether.
It would be another great advance in the art if an improved driving method for electrostatic actuators could be provided for use with any actuator and configuration of the physical components thereof.
Other advantages will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention comprises an actuator drive producing a compounded AC signal with three different sections, called Section I or the Rise Section, Section II or the Normal Section, and Section III or the Fall Section. The normal section will serve as the actual force generating section while the rise and fall sections will serve as transition sections between the zero signal and the nominal amplitude of the AC signal. This drive results in an overall increase of the actuator life time as a result of reducing charging and reducing the mechanical impact at the contact between actuator parts. The signals used in the different sections may be square-wave, sine wave, triangular wave or mixed signals.
The device is used in electrostatic actuators that have at least one pair of actuator plates and electrodes for conducting a voltage potential thereto. At least one of the pair is movable with respect to the other. The plates are positioned to move upon application of a voltage potential through the electrodes. A driving means provides a voltage potential to the electrodes to cause the movement of the at least one plate.
The driving means of this invention produces a compounded AC signal with the following structure. The signal in section II or the nominal section is a square wave AC signal. This signal is able to generate a steady electrostatic force. The amplitude and the frequency of this signals are selected according to the mechanical and electrical characteristics of the particular application. An example of the above signal would be: amplitude of the square wave AC signal 60 V peak-to-peak and frequency of 250 Hz. In some applications, where life time requirements are modest (several hundreds of thousands of actuations cycles) this section of the signal can stand alone.
The signals in sections I and III do not have to generate a significant electrostatic force but to ensure a slow increase/decrease of the amplitude of the electrical signal. The increase/decrease can be considered slow if the time for the signal to go from zero to its maximum value is at least 10 to 100 times longer than the mechanical response time of the actuator. In this way, the displacement of the actuator will be able to follow the applied voltage. Signals satisfying these requirements are the amplitude modulated square wave, sine wave or triangular wave signals. The amplitude modulation can follow a desired time dependence such as, but not limited to, a linear exponential or sinusoidal function. The number of cycles of the AC signal in sections I and III can be varied according to the response time and power requirements of the particular application. A preferred embodiment is to have more than 20 cycles.
When sine wave or triangular wave signals are used in section I and/or III of the compounded signal, that section can be reduced to a quarter of a full AC cycle. This allows a significant reduction of the response time and of the power consumption of the device, keeping at the same time the advantages of reduced mechanical impact and reduced charging.
The preferred circuit produces an actuation cycle having an amplitude increasing from zero to the desired final value through a plurality of individual periods of the basic AC signal and an amplitude decreasing from a nominal final value to zero through a similar plurality of individual periods of the basic AC signal.
The detailed structure of the rise/fall sections and the frequency of the AC signal can be modified over a wide range of values to fit the speed and power requirements of the specific application. It is intended that the specific frequency of the AC signal and the times of the rise/fall sections be adjusted to minimize stiction between said actuator plates.